Photo of the rumbleometer stuck in the new lava
flow. Image taken by the remotely operated vehicle ROPOS.Virtual view of the rumbleometer
and ROPOS. Image from the Rumbleometer
Virtual site animation.Rumbleometer deployment prior eruption.Animation
of the lava flow and the rumbleometer's ride up and down on top of the new
lava flow.

The
Rumbleometer story

Soon
after the earthquake swarm began on January 25th, 1998, a lava flow
emerged from an eruptive fissure along Axials upper south rift
zone. It just so happened that a seafloor monitoring instrument called
a "rumbleometer" (or VSM) was in its path. The instrument
became trapped in the lava flow, but amazingly it survived and was successfully
rescued a year and a half later. The data recovered from the rumbleometer
reveal an extraordinary sequence of events and for the first time document
the dynamics of a seafloor eruption on the mid-ocean ridge.

The
data from the rumbleometer that reveals the most about the eruption is
from the pressure sensor, which recorded vertical movements [changes in
depth] of the instrument. The pressure data show that the lava flow first
spread out as a thin sheet under the instrument. Then the lava flow stopped
speading outward and began to thicken or inflate upward with a solid crust
over a molten interior. During this inflation stage the rumbleometer was
actually lifted upward over 3 meters (9 feet) by the lava flow. Soon thereafter
the eruption began to wane and lava drained back into the eruptive vent,
and the rumbleometer was lowered back down, ending up only 0.5 meters
(18 inches) higher than it started. Perhaps most remarkably, all this
took place in the span of only 2 hours. This is the first time that a
submarine eruption has actually been caught in the act, all because an
instrument was in the right place at the right time, and managed to survive!

The
rumbleometer was in remarkably good condition after it was rescued. Only
the yellow plastic "hard-hats" that protect the glass ball flotation
were melted in a few
spots where they had come in contact with the lava. Amazingly, a temperature
sensor on board the rumbleometer only recorded a 4°C (7°F) increase
in temperature during the eruption! Even though the lava was erupted at
a temperature of about 1200 °C (2200°F), the instrument was kept
relatively cool because it was bathed in frigid seawater at 2°C [36°F]
which dissipated most of that heat. Also the lava quickly forms a solid
crust over its molten surface, which greatly insulates the heat below.